SENIOR NOTES PAYABLE AND OTHER DEBT - Summary of Senior Notes Payables and Other Debt (Details) CAD in Thousands, $ in Thousands	Mar. 31, 2017 CAD	Mar. 31, 2017 USD ($)	Dec. 31, 2016 CAD	Dec. 31, 2016 USD ($)	Sep. 30, 2016 CAD
Debt instruments
Principal amount outstanding		$ 12,020,558		$ 11,190,914
Deferred financing costs, net		(64,086)		(61,304)
Unamortized fair value adjustment		19,708		25,224
Unamortized discounts		(32,447)		(27,508)
Senior notes payable and other debt		11,943,733		11,127,326
Unsecured revolving credit facilities
Debt instruments
Principal amount outstanding		170,731		146,538
Unsecured revolving credit facilities | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 152,700		CAD 146,500
1.250% Senior Notes due 2017
Debt instruments
Principal amount outstanding		$ 300,000		300,000
Interest rate	1.25%	1.25%
2.00% Senior Notes due 2018
Debt instruments
Principal amount outstanding		$ 700,000		700,000
Interest rate	2.00%	2.00%
Unsecured Term Loan Due 2018
Debt instruments
Principal amount outstanding		$ 200,000		200,000
Unsecured term loan due 2019
Debt instruments
Principal amount outstanding		372,042		371,215
4.00% Senior Notes due 2019
Debt instruments
Principal amount outstanding		$ 600,000		600,000
Interest rate	4.00%	4.00%
3.00% Senior Notes, Series A due 2019
Debt instruments
Principal amount outstanding | CAD	CAD 300,503		297,841
Interest rate	3.00%	3.00%
2.700% Senior Notes due 2020
Debt instruments
Principal amount outstanding		$ 500,000		500,000
Interest rate	2.70%	2.70%
Unsecured term loan due 2020
Debt instruments
Principal amount outstanding		$ 900,000		900,000
4.750% Senior Notes due 2021
Debt instruments
Principal amount outstanding		$ 700,000		700,000
Interest rate	4.75%	4.75%
4.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding		$ 600,000		600,000
Interest rate	4.25%	4.25%
3.25% Senior Notes due 2022
Debt instruments
Principal amount outstanding		$ 500,000		500,000
Interest rate	3.25%	3.25%
3.300% Senior Notes due 2022
Debt instruments
Principal amount outstanding | CAD	CAD 187,815		186,150
Interest rate	3.30%	3.30%
3.125% Senior Notes due 2023
Debt instruments
Principal amount outstanding | CAD	CAD 400,000		400,000
Interest rate	3.125%	3.125%
3.100% Senior Notes due 2023
Debt instruments
Principal amount outstanding | CAD	CAD 400,000		0
Interest rate	3.10%	3.10%
3.750% Senior Notes due 2024
Debt instruments
Principal amount outstanding		$ 400,000		400,000
Interest rate	3.75%	3.75%
4.125% Senior Notes, Series B due 2024
Debt instruments
Principal amount outstanding | CAD	CAD 187,815		CAD 186,150
Interest rate	4.125%	4.125%
3.500% Senior Notes due 2025
Debt instruments
Principal amount outstanding		$ 600,000		600,000
Interest rate	3.50%	3.50%
4.125% Senior Notes due 2026
Debt instruments
Principal amount outstanding		$ 500,000		500,000
Interest rate	4.125%	4.125%
3.25% Senior Notes due 2026
Debt instruments
Principal amount outstanding		$ 450,000		450,000
Interest rate	3.25%	3.25%
3.850% Senior Notes due 2027
Debt instruments
Principal amount outstanding		$ 400,000		0	CAD 400,000
Interest rate	3.85%	3.85%
6.90% Senior Notes due 2037
Debt instruments
Principal amount outstanding		$ 52,400		52,400
Interest rate	6.90%	6.90%
6.59% Senior Notes due 2038
Debt instruments
Principal amount outstanding		$ 22,973		22,973
Interest rate	6.59%	6.59%
5.45% Senior Notes due 2043
Debt instruments
Principal amount outstanding		$ 258,750		258,750
Interest rate	5.45%	5.45%
5.70% Senior Notes due 2043
Debt instruments
Principal amount outstanding		$ 300,000		300,000
Interest rate	5.70%	5.70%
4.375% Senior Notes due 2045
Debt instruments
Principal amount outstanding		$ 300,000		300,000
Interest rate	4.375%	4.375%
Mortgage loans and other
Debt instruments
Principal amount outstanding		$ 1,717,529		$ 1,718,897
Unsecured term loan
Debt instruments
Principal amount outstanding | CAD	CAD 572,000
Unsecured term loan | Canada, Dollars
Debt instruments
Principal amount outstanding | CAD	CAD 93,500